<div class="container">

  <div class="element">
    <p>
      My line height is set using a unit value 120%. My font size is 18px. The computed value that will be inherited by my child here is 21.6px.
    </p>
    <div class="child">
      I am the child. My font size is bigger than my parent's—it's set to 28px. The line height I inherited from my parent is too small for my text, so my lines overlap. :(
    </div>
  </div>
  <div class="element unitless">
    <p>
      My line height is set using a unitless value 1.5. My font size is 18px. The computed value that will be inherited by my child here is 27px.
    </p>
    <div class="child">
      I am the child. My font size is bigger than my parent's—it's set to 28px. The line height I inherited from my parent is 1.5 because it's unitless, so my own line height is computed based on my own font size, which is good.
    </div>
  </div>
</div>
body {
  color: #555;
  font-size: 1.1em;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
}
hr {
  margin-top: 50px;
}
.container {
  margin: 40px auto;
  max-width: 700px;
}
.element {
  padding:20px;
  background-color: #009966;
  color: white;
  margin-bottom: 30px;

  font-size: 18px;
  line-height: 120%;
} 
.child {
  font-size: 28px;
}
.element.unitless {
  line-height: 1.5;

  background-color: purple;
}